High bandwidth - time product spin echo system

ABSTRACT

A system for the generation of a spin-echo signal utilizing a paramagnetic sample of titanium dioxide (rutile) selectively doped with nickel wherein the crystalline electric field of the rutile doped with nickel splits the energy level of the nickel at a predetermined frequency (8.25GHz) and spin-echo signals are produced without the requirement of an external magnetic field.

United States Patent Bozanic et a1.

[ Nov. 27, 1973 HIGH BANDWIDTH TIME PRODUCT SPIN ECHO SYSTEM Inventors:Donald A. Bozanic; Dickron Mergerian, both of Baltimore; Ronald W.Minarik, Lutherville; Peter H. Pincoffs, West Severna Park, all of Md.

Assignee: Westinghouse Electric Corporation Pittsburgh, Pa.

Filed: Apr. 30, 1970 Appl. No.: 33,201

Int. Cl. G0ln 27/78 Field of Search 324/05 R, 0.5 A; 340/173 NIReferences Cited UNITED STATES PATENTS 6/1972 Bozanic 324/05 6/1967Kaplan 340/173 N1 OTHER PUBLICATIONS D. A. Bozanic, D. Mergerian and R.W. Minarick-E- U.S. Cl 324/05 R, 340/173 NI lectron Spin-EchoMeasurements of E Centers In Crystalline Quartz-Phys. Rev.Letters21(8)-8/19/68, pp. 541-542.

H. J. Gerritsen, S. E. Harrison, and H. R. Lewis-- Chromium DopedTitania as a Maser Material-Jour. of App. Phys. 31(9)-9/60-pp.1566-1571.

D. E. Kaplan Zero Field ESR Spectra and Relaxation of Ce in CaF at HighConcentrations-Bull. Am. Phys. Soc. 1963 pg. 468.

Primary Examiner-Michael J. Lynch Att0rney-F. H. Henson, E. P. Klipfeland J. L. Wiegreffe [57] ABSTRACT A system for the generation of aspin-echo signal utilizing a paramagnetic sample of titanium dioxide(rutile) selectively doped with nickel wherein the crystalline electricfield of the rutile doped with nickel splits the energy level of thenickel at a predetermined frequency (8.25GHz) and spin-echo signals areproduced without the requirement of an external magnetic field.

7 Claims, 2 Drawing Figures CIRCULATOR 26 CRYOSTAT NICKEL DOPED RUTILESAMPLE 28 PATENl mnvzmza 3,775,670

H? I" I 2 in rm ECHO BALANCED so MHZ 2 ND MIXER IF DETECTOR 32 DOUBLE IT PULSER :Fllfi RF DiODE CIRCULATOR SWITCH NICKEL DOPED RUTILE SAMPLE 28I HIGH BANDWIDTH TIME PRODUCT SPIN ECI-IO SYSTEM BACKGROUND OF THEINVENTION 1. Field of the Invention The subject invention is directed tospin-echo" systerns utilizing a paramagnetic sample located in microwavetransmission line means operated at cryogenic temperatures wherein theapplication of a first and a second RF pulse in the sample gives rise toa spin-echo signal which appears at a time following the second RF pulsecorresponding to the time interval between the two RF pulses and moreparticularly,'to a spin-echo system utilizing a sample comprised oftitanium dioxide (rutile) doped with iron group transition metal ions.

The spin-echo phenomenon normally exists when a paramagnetic sample islocated in a resonant cavity situated between a homogeneous DC magneticfield such that when a first or input RF pulse having a frequency equalto the characteristic or Larmor frequency of the sample is applied atright angles to the DC magnetic field whereupon a torque is applied tothe magnetic moment which causes it to be tipped away from the directionof the magnetic field. The angle of tipping, that is the angle betweenthe moment and the direction of the field is proportional to themagnitude of the field and the time during which the RF pulse exists.Upon release of the displacing force, the spinning electrons urged againtowards realignment by the force of the magnetic field rotate or precessabout the field in much the same manner as a tipped gyroscope. When asample is subsequently subjected to another or recall" RF pulse alsodirected normal or transverse to the magnetic field, the samplespontaneously develops a magnetic field of its own which is also normalto the magnetic field and which rotates about the latters direction. Thestrength of the rotating field builds up to a maximum and then decayswhich is then detected as an electrical pulse called a spin-echo"signal.

2. Description of the Prior Art Pulsed nuclear induction spin-echosystems are well known to those skilled in the art. An example of such ateaching is found in US. Pat. No. 2,887,673 issued to E. L. Hahn.Further development in the art resulted in electron spin-echo systems,an example of which is disclosed in US. Pat. No. 3,129,410 issued to P.P. Sorokin. The difference between electron and nuclear spin systems isthe difference in the operating frequency encountered. This isattributed to the smaller mass of the electron in comparison to theproton in the nucleus.

The precessional or Larmor frequencies of electrons therefore lie in themicrowave range.

Still a later development has been the recent discovery of a zero-fieldspin-echo signal reported on Mar. 31, 1968 by D. E. Kaplan in a finalreport by the Lockheed Research Laboratory on Contract Number No.2541(). This publication indicated that an electron spin-echo signal wasobserved at zero magnetic field utilizing CaF doped with O.l 0.5 percentof Ce. Line widths of 2OMHZ having a phase memory time of 1 microsecondwere observed.

Additionally, a system for the generation of an exchange echo signalsimilar to a spin-echo signal has been developed comprising a veryheavily doped sample of paramagnetic material such as rutile. The rutilesample is doped with iron group transition metal ions producingparamagnetic defect centers in the order of l X 10 to l X 10 defectscenters per cm wherein no magnetic field of any kind is necessary forthe production of the echo signals and wherein echo bandwidthsconsiderably greater than those generally associated with spin-echoesare achievable. This concept is taught in co-pending US. Pat.application Ser. No. 845,406 filed on July 28, 1969, now US. Pat. No.3,671,855, in the names of the inventors of the subject invention andbeing entitled Broadband Zero Field Exchange Echo System." The efiectproduced therein is believed due to a series of lines which appear dueto the iron group transition ions which are influenced by other ionslocated near each other and coupled through exchange interaction.

Where a spin-echo microwave memory system is desired, the bandwidth time(BT) product also often referred to as the pulsed compression ratio mustbe comparable with conventional memory devices. The largest bandwidthtime product previously observed in a spinecho system was 250 which wasexperimentally observed and reported by Clark Mollenhauer and Owen Lewisof the Syracuse University Research Corporation in Microwaves, Volume 8,No. 10, page 12, Oct. 1969. The paramagnetic sample employed wasphosphorus doped silicon operating at liquid helium temperatures.

SUMMARY The present invention has for its object the provision of a veryhigh bandwidth time (BT) product spinecho system by utilizing nickeldoped titanium dioxide (rutile) selectively doped to control the spinlattice relaxation time and phase memory time. When rutile doped withnickel is utilized as a spin-echo sample and operated at a frequency of8.25GHz, the crystalline electric field of the sample itself splits theenergy levels of the nickel without the need for an externally appliedDC magnetic field. The subject sample is located in a microwavetransmission line operated at cryogenic temperatures. Means areadditionally included for coupling a first or input and a second orrecall" microwave pulse at a predetermined frequency such as 8.25GHzinto the sample whereupon a spin-echo signal is produced at a timesubstantially equal to the interval occurring between the input andrecall pulse.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a firstembodiment of electrical apparatus for practicing the subject invention;and

FIG. 2 is a fragmentary perspective view of the microwave transmissionline adapted to hold the nickel doped rutile spin-echo sample.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings,FIG. 1 discloses, inter alia, a microwave source 10 operable in theX-band range of microwave frequencies and preferably but notexclusivelyat 8.25GHz. The microwave source 10 is coupled to acontrolled RF diode switch 12 by means of a first microwave signalwaveguide path including the level set attenuator 14. The RF diodeswitch 12 is controlled by a double ,pulser electronic circuit 16 whichis operable to produce a pair of gate signals separated bya-predetermined time interval T for producing an output of the diodeswitch 12 which comprises a pair of microwave pulses corresponding tothe input" pulse Pr and the "recall" pulse P of a spinecho system. Thepulses P, and P are coupled to a circulator 18 by means of a secondwaveguide path including a second level set attenuator 20 whereupon themicrowave pulses P, and P, are transferred to a microwave transmissionline means comprising a waveguide section 22. The waveguide section 22is terminated in a waveguide load comprising a short 24 and is immersedin a cryostat 26 which contains a cryogenic material such as liquidhelium for operation at extremely low temperatures (4K and below, e.g.,l.2l().

The waveguide section 22 at the termination 24 contains a spin-echosample 28 comprised of paramagnetic material having the followingproperties: (a) the host material contains no nearest-neighbor elementsto the echo-producing paramagnetic defect which possess nuclear spins;(b) the resonance transition of the paramagnetic defect permits zeromagnetic field operation at some microwave operating frequency whichpermits relatively high BT products to be achieved; (c) the phase memorytime can be controlled by the doping concentration; and (d) the materialexhibits a spinlattice relaxation time comparable to the memory cyclerepetition period obtained in conventional memory systems.

Based on these considerations, a sample of titanium dioxide or rutiledoped with non-Kramers" ions such as nickel ions (Ni is utilized as thespin-echo sample 28. Various doping levels can be selectively obtainedby completely immersing the rutile host lattice in a nickel oxide (NiO)powder and heating it to 1,l C in air for various lengths of time. Thenickel ion which is in the atomic state 2+ is employed as theechoproducing defect. The nearest atomic neighbors to this ion which areoxygen ions, however, contain no nuclear spins. Secondly, thecrystalline electric field of the rutile doped with nickel splits theenergy levels of the nickel at an operating frequency of 8.25GHzproducing what is referred to as zero field splitting of the spinseliminating the need for an external magnetic biasing field. When thepulses P, and P, are coupled to the nickel doped sample 28 which is at atemperature below 4K and more particularly at 1.2"K which can beobtained by suitable modification of the cryostat 26, a spin-echo"signal is produced which is coupled back along the waveguide 22 throughthe circulator 18 and to an output microwave transmission line 30including a wave meter 32. The spin-echo signal is applied to thebalanced mixer 36 which receives a local oscillator sig nal from theoscillator 38 by means of the waveguide path including the variableattenuator 40. The output of the mixer 36 comprises an IF signal of thetwo microwave pulses P, and P, used to generate the spin-echo signal inaddition to the echo signal itself. This IF signal is coupled to an IFamplifier 42 where it is then fed into a second detector 44 whichproduces a video signal output of the pulses concerned which are thenviewed on a suitable indicator 46 which may be, for example, anoscilloscope.

The following experimental results were obtained for different typicalrutile samples having various doping levels as obtained by varying thedoping time with Ni0 powder and when the samples were operated at1.2Kelvin and at the zero field operating frequency of 8 25 GHz:

P hase Electron Paramagnetic M empry Sam ple Doping Time Resonance TuneI ll) Minutes 150MHz SOOpsec.

2 Minutes -l50Ml-lz 600 sec.

3 3 Days 2,0GHz l 10 psec.

4 4 Days 2,0GHZ psec.

TABLE I It becomes evident that the phase memory time is dependent uponthe doping concentration and can be increased by selectively loweringthe doping concentration; however. it may be desirable in some cases toraise the doping concentration in order to increase the paramagneticbandwidth by creating exchange lines such as taught in co-pending U.S.Ser. No. 845,406 referenced above. Although the phase memory timedecreases for increased doping concentrations the increase in bandwidthsmay be in higher proportion so as to yield a higher overall BT product.

Additionally, the spin-lattice relaxation time is in the order of 10 Msec. at 1.2Kelvin and therefore permits operation with a lOQ Hz PRF.This can be increased continuously by raising the temperature so thatfor example a 20KHz PRF can be obtained at 4.2K. Normally, the value ofthe spin-lattice relaxation time at l.2Kelvin for most other spin-echomaterials is in the order of seconds and as a result does not permit auseable PRF. However by utilizing nickel doped rutile a material isselected which possesses a non-Kramers ion wherein an even number ofelectrons appear in the outer shell so that there are no unpaired spinsand it is a characteristic of these ions that they have a much fasterspin-lattice relaxation rate in a zero field which occurs at 8.25GHz fornickel doped rutile.

By employing suitably nickel oxide doped titanium dioxide a BT productof 20,000 can be obtained for a fundamental resonance line mode(8.25GHz) when samples 1 and 2 are utilized but a BT product of 200,000can be obtained in an exchange" line mode such as would be the case whensamples 3 and 4 are utilized. Still significant, however, is the factthat the doping with a non-Kramers ion permits a useable PRF. inaddition the spin-echo material is easily produced by simply diffusingNil) into Ti0 A relatively low insertion loss, e.g., approximately 24dbcan be obtained at 1.2K in zero magnetic field and because of its highdielectric constant possesses such excellent coupling to the microwavesignals P and P, that it need only be inserted in a section of thewaveguide.

Having thus described by wave example what is considered at present tobe the preferred embodiment of the subject invention,

We claim as our invention:

1. A spin-echo system comprising in combination:

means for generating at least first and second RF microwave pulsesseparated by a predetermined time interval;

waveguide transmission line means coupled to said first and said secondRF microwave pulse;

a spin-echo sample positioned in a substantially zero steady statemagnetic field, comprising a rutile sample selectively doped with nickelions with the doping level being selected to give the desired bandwidthand phase memory time, said sample being located in said waveguidetransmission line means, and being responsive to said first and saidsecond microwave pulse to produce a spin-echo signal; and

means surrounding said spin-echo sample for reducing the temperature ofsaid sample to a predetermined cryogenic temperature.

2. The invention as defined by claim 1 wherein said cryogenictemperature is substantially in the region of from l.2K to 4.2K.

3. The invention as defined by claim 2 and wherein said microwavetransmission line means comprises a section of waveguide terminated in aload and including means for locating said spin-echo sample therein.

4. The method of increasing the dynamic range and repetition rate of aspin-echo signal comprising the steps of:

positioning a rutile spin-echo sample doped with nickel ions, theconcentration of said nickel ions being selected to give the desiredbandwidth and phase memory time within a microwave transmission line,said sample being subjected to substantially zero steady state magneticfield;

lowering the temperature of said spin-echo sample to a predeterminedcryogenic temperature; and

applying a first and a second pulse of RF energy to said spinecho samplein a predetermined timed relationship to produce a spin-echo signal.

5. The method as defined by claim 4 wherein said sample is doped byimmersing said sample in a nickel oxide powder and heating it to l l00Cin air for a predetermined length of time, said time determining thedoping concentration.

6. The method as defined by claim 5 wherein said step of applying afirst and a second pulse of RF energy comprises applying a first and asecond pulse of RF energy at an operating frequency in the region of andincluding 8.25GHz.

7. The method as defined by claim 6 wherein said predetermined cryogenictemperatures is in the range including l.2Kelvin and 4.2Kelvin.

2. The invention as defined by claim 1 wherein said cryogenictemperature is substantially in the region of From 1.2* K to 4.2* K. 3.The invention as defined by claim 2 and wherein said microwavetransmission line means comprises a section of waveguide terminated in aload and including means for locating said spin-echo sample therein. 4.The method of increasing the dynamic range and repetition rate of aspin-echo signal comprising the steps of: positioning a rutile spin-echosample doped with nickel ions, the concentration of said nickel ionsbeing selected to give the desired bandwidth and phase memory timewithin a microwave transmission line, said sample being subjected tosubstantially zero steady state magnetic field; lowering the temperatureof said spin-echo sample to a predetermined cryogenic temperature; andapplying a first and a second pulse of RF energy to said spin-echosample in a predetermined timed relationship to produce a spin-echosignal.
 5. The method as defined by claim 4 wherein said sample is dopedby immersing said sample in a nickel oxide powder and heating it to1100* C in air for a predetermined length of time, said time determiningthe doping concentration.
 6. The method as defined by claim 5 whereinsaid step of applying a first and a second pulse of RF energy comprisesapplying a first and a second pulse of RF energy at an operatingfrequency in the region of and including 8.25GHz.
 7. The method asdefined by claim 6 wherein said predetermined cryogenic temperatures isin the range including 1.2* Kelvin and 4.2* Kelvin.